A thin-film transistor (TFT) is used as a transistor for driving the display device of a mobile device such as a mobile phone, notebook-sized personal computer, tablet computer or PDA because its element area is small and it is space-saving.
In the past, most of thin-film transistors have been made of a silicon-based semiconductor material, typically amorphous silicon or polycrystalline silicon. However, a thin-film transistor which uses a silicon-based semiconductor material has a problem that its sub-threshold slope is as large as about 200 mV/decade and if this thin-film transistor is used to form a circuit, operation at a low voltage of about 1 to 3 V, at which an ordinary large-scale integrated circuit operates, is difficult. In addition, since its off-current is large, it has a problem that it is difficult to decrease the standby power consumption.
A solution to these problems is to produce a fully depleted state while the thin-film transistor is off, but in the case of a thin-film transistor which uses a silicon-based semiconductor material, for a reason related to the manufacturing process, it is not easy to produce a fully depleted state.
For these reasons, in recent years, research and development of thin-film transistors which use wide bandgap oxide semiconductor for a channel layer has been conducted actively.
For example, it is reported on pages 77 to 80 of Nonpatent Literature 1 (IEDM Tech. Dig., (2008)) that in a thin-film transistor using In—Ga—Zn—O (IGZO) for a channel layer, if the channel layer thickness is as thin as about 10 nm, the sub-threshold slope is not more than 100 mV/decade.
Also, it is reported on pages 73 to 76 of the above Nonpatent Literature 1 and Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2009-170905) that when two or more oxide semiconductor layers are stacked, the on-current and field effect mobility are twice or more higher than in a thin-film transistor with a single-layer channel structure.
Furthermore, since a thin-film transistor which uses oxide semiconductor for a channel layer can be manufactured at low temperature, it offers an advantage that a device can be easily formed on a glass substrate or flexible substrate and a nonconventional new device can be manufactured at low cost. In addition, by taking advantage of the transparency as a feature of oxide semiconductor, it can be used not only for a display device but also for various electronic devices such as an RFID tag.